1. Field of the Invention
The present invention relates to an outboard motor detachably mounted at a stern for use in propelling a boat or ship, as well as an engine mounted in the outboard motor. The engine according to the present invention can be utilized not only as an engine for the outboard motor, but also as a general-purpose engine.
2. Description of the Related Art
In a common type of the outboard motor of the related art, an engine (a vertical engine) having a flywheel provided at an upper end of a vertically directed crankshaft protruding from an engine block is mounted in an outboard motor body case which is mounted to a boat body through an antivibration mount. Such types of the outboard motors are disclosed, for example, in Japanese Patent Application Laid-open Nos. 191610/87, 192917/88 and 192918/88.
In these outboard motors, a ring gear is mounted around an outer periphery of the flywheel, and a starter motor is mounted above a side of the engine and meshed with the ring gear. A driving pulley of a valve-operating belt drive transmission is provided at an end of the crankshaft adjacent and below the flywheel.
Typically, an ignition power source coil and a charging power source coil are accommodated in the flywheel to form a dynamo and hence, the flywheel has a downwardly-turned bowl-like shape.
In such a prior art outboard motor, the heavy flywheel having a large inertial moment, which largely influences the determination of the position of the center of gravity of the engine, is spaced farther upwardly from the antivibration mount. The crankshaft end opposite from the flywheel is coupled to a driving shaft for transmitting a driving propeller. Therefore, factors of a torsional vibrational effects exert a great influence on the selection of the antivibration mount and hence, the selection of the antivibration mount must be taken into special consideration.
In addition, not only the flywheel but also a starter must be mounted above the engine. Therefore, the position of the center of gravity of the engine is high, which increases the moment required during tilting-up of the outboard motor, and also limits the freedom of the disposition of other auxiliaries, especially in the case of multi-cylinder engine, the disposition of an electrical equipment box for accommodating a CDI unit and a plurality of coils, other auxiliaries such as intake system auxiliaries or the like.
Further, in a 4-cycle engine used in the outboard motor, the driving pulley of the wrapping type transmission is provided as a valve operating device at the crankshaft end adjacent the flywheel. The crankshaft end, however, requires a large diameter for mounting the flywheel. Therefore, the diameter of the driving pulley must be increased and as a result, a driven pulley adjacent a camshaft is also increased in size and has a shape occupying an area near an upper portion of a cylinder head, bringing about an increase in size of an upper portion of a rear end of an engine cover spaced from a tilting shaft. However, this portion of the engine cover is liable to interfere with a boat body structure, when the outboard motor is turned upwardly about the tilting shaft and hence, the unnecessary increase in size of this portion is undesirable and inconvenient even in respect of a moment required for the turning of the outboard motor.
A lower portion of the outboard motor body case is formed to be narrow in order to reduce the underwater resistance of a submerged portion of the case to the utmost and to provide a reduction in weight. Therefore, an engine having a good mountability to such outboard motor body case is desired.
In Japanese Utility Model Application Laid-open Nos. 21509/91 and 23609/91, there has been proposed an engine in which a crankshaft is oriented vertically and a flywheel is provided at a lower end of the crankshaft protruding from an engine block. Such an engine includes a transmission connected to that lower end of the crankshaft which is provided with the flywheel. Thus, this engine cannot be applied directly as an engine for use in the outboard motor, and such prior art devices do not suggest any means capable of solving problems inherent in the engine of the above-described type for use in the outboard motor.